ARTICLE pubs.acs.org/JPCC
Scanning Kelvin Probe Study of (Oxyhydr)oxide Surface of Aluminum Alloy †,‡ € Ozkanat, € O. B. Salgin,§ M. Rohwerder,§ J. M. C. Mol,‡ J. H. W. de Wit,‡ and H. Terryn*,†,^ †
Materials innovation institute (M2i), Mekelweg 2, 2628 CD Delft, The Netherlands Department of Materials Science and Engineering, Delft University of Technology, Mekelweg 2, 2628 CD Delft, The Netherlands § Max-Planck-Institut f€ur Eisenforschung GmbH, Max-Planck-Strasse 1, 40237 D€usseldorf, Germany ^ Research Group Electrochemical and Surface Engineering, Vrije Universiteit Brussel, Pleinlaan 2, B-1050 Brussels, Belgium ‡
bS Supporting Information ABSTRACT: In this paper, the work function, referenced as electrode potential vs SHE, of modified (oxyhydr)oxide surfaces of AA1050 was studied upon exposure to different relative humidity (40% and 90%) and aging (40 and 240 min) by utilizing scanning Kelvin probe (SKP). Besides reference samples, different pretreatments (acid, alkaline, and immersion in boiling water) were applied in order to create variations in the surface properties (i.e., hydroxyl content, oxide thickness, surface morphology). The observed trend in the potential was correlated to the hydroxyl fraction and the oxide thickness of the modified surfaces, determined by X-ray photoelectron spectroscopy and visible spectroscopic ellipsometry , respectively. Fourier transform infrared spectroscopy was also used to investigate the chemical composition of the modified surfaces. It is presented how the surface potential of aluminum (oxyhydr)oxide is influenced by the oxide properties, such as hydroxyl fraction and thickness. This characterization work on bare (oxyhydr)oxide aluminum surfaces is a preliminary study and opens a way to understand the contribution of oxide properties to the interfacial bonding in the presence of organic coating.
’ INTRODUCTION Aluminum alloys, prone to corrosion-induced degradation, are generally in need of protection by organic coatings1 3 which also provide various surface properties such as color, wear resistance, and formability. Interfacial bonds at such polymer/(oxyhydr)oxide/aluminum joints have to withstand high mechanical forces and corrosive attack to protect the functional properties of the coated metals in its intended service application conditions. Therefore, it is crucial to control and to understand the adhesion mechanism originating at polymer/(oxyhydr)oxide/aluminum interfaces in order to achieve the long-term stability of these composites.4 Adhesion at polymer/aluminum interfaces is mainly governed by the adsorption theory, taking into account covalent, polar, and acid base interactions.4 Therefore, the resistance of these interfaces is controlled not only by the adhesion-promoting functional groups in organic coating but also by the interrelated properties of passive layers formed on aluminum surfaces, e.g., hydroxyl content,5 thickness, surface morphology6,7 electronic structure,8 and chemical composition.9 Those properties of passive layers rely heavily on pretreatments of aluminum and also might be extremely sensitive to the small changes in the environmental conditions, e.g., humidity10,11 and aging.11,12 For instance, in the case of repeatability of coating application, humidity remains as almost a universal problem in various industrial areas such as packaging, coastal, marine, and aerospace, where coating must be applied within the humidity range at which moisture will not condense on the metal surface.13 Scanning Kelvin probe (SKP) is a suitable technique to monitor Volta potential changes on bare metals. The chemical r 2011 American Chemical Society
and/or structural changes in insulating or semiconducting oxide layers formed on base metals are directly reflected in the potential measured by SKP.14 Another advantage of SKP is its ability to probe the buried polymer/metal interfaces making it established in corrosion science as a nondestructive technique to study the fundamental processes of corrosion, e.g., ion mobility15,16 and delamination of coatings on different substrates such as iron oxides,16,17 zinc oxides,18 and aluminum oxides.19 In such delamination studies, different parameters determining the resistance of coated metal systems, such as polymer composition,17 oxide surface energy,16 and atmospheric humidity,18 have been investigated. In order to understand how the delamination process is affected by the surface properties prior to the coating application, the surface potential can be studied on modified bare (oxyhydr)oxide surfaces. Recently, Yasakau et al.20 studied the Volta potential difference (VPD) on various (oxyhydr)oxide surfaces of 99.999% pure Al substrates by scanning Kelvin probe force microscopy (SKPFM), and they observed an increase of VPD with increasing oxide thickness on anodized surfaces. This correlation was attributed to the changes in the electrical properties of the oxides, induced by charges embedded during anodization, which result in a gradient in the charge distribution throughout the oxide with the highest concentration of cations (Al3+) and anions (O2 ) at metal/oxide and oxide/electrolyte, Received: June 14, 2011 Revised: October 14, 2011 Published: November 17, 2011 1805
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The Journal of Physical Chemistry C respectively.20 Furthermore, Hausbrand et al.14 explained how the semiconducting properties of oxides—in addition to the thickness—determine the potentials measured by SKP. Nevertheless, in these works the effect of hydroxyl fraction and humidity on potential was not studied. X-ray photoelectron spectroscopy (XPS), performed in an ultrahigh vacuum (UHV) chamber, has been proved as a successful technique to obtain quantitative information on the amount of hydroxyls in the outer oxide layer.3,5,21 In an earlier study,21 different aluminum oxide layers were investigated by using XPS. From peak fitting of the O1s peak, a linear relation was determined between the O/Al atomic ratio and the hydroxyl fraction of the studied oxide layers. Yamamoto et al.22 performed in situ XPS studies of water chemistry on Cu(110) at near ambient conditions and in UHV at low temperatures, and they reported a good agreement in the binding energies, H2O:OH ratios, and stability order of phases. In this work, we studied how the surface properties of AA1050 are reflected in the potential prior to coating application. First, electropolished AA1050 was modified to create variations in the surface properties (e.g., hydroxyl content, oxide thickness, surface morphology) by applying different pretreatments (acid, alkaline, and immersion in boiling water besides the reference surface). Then, the potentials of the modified (oxyhydr)oxide surfaces were acquired at two different relative humidity (RH) levels (40% and 90%) for different exposure durations (40 and 240 min) by utilizing SKP. The acquired potential values were correlated to the hydroxyl fraction determined by the curve fitting of the O1s peak (obtained by XPS) and to the oxide layer thickness obtained by visible spectroscopic ellipsometry (VISSE). Moreover, supportive chemical information was obtained by Fourier transform infrared (FTIR) spectroscopy. This fundamental investigation on bare aluminum surfaces has been planned as a preliminary step of a series of studies aiming to reveal the factors affecting the durability of coated aluminum systems.
’ EXPERIMENTAL METHODS Material and Sample Preparation. Two millimeter thick AA1050 sheet (Salomon’s Metalen; Groningen, Netherlands) was cut into pieces of 20 50 mm. First, the sample was immersed in a 25 g/L aqueous solution of NaOH at 70 C for 1 min. Then, it was rinsed for 10 s with deionized water. This step was followed by ultrasonic cleaning in deionized water for 2 min. After drying, in order to obtain a flat surface and to remove the oxides of the as-received sheet, the degreased surface was electropolished for 6 min in a 80 vol % ethanol 20 vol % perchloric acid solution with a current density of 70 mA/cm2, which was followed by 10 s rinsing with deionized water; thereafter, it was ultrasonically cleaned in deionized water for 2 min. After the sample was dried with compressed nitrogen, it was ultrasonically rinsed in ethanol for 10 min. A set of samples, tested after this step, was referred as reference. In order to prepare the modified (oxyhydr)oxide surfaces, samples further received one of the three pretreatments: acid, alkaline, and immersion in boiling water. For acid pretreatment, the sample was immersed into 30 vol % HNO3 solution (chemically pure) in deionized water for 30 s at room temperature. This was followed by 3 min rinsing with running deionized water and drying with compressed nitrogen. For alkaline-pretreatment, the sample was immersed in a pH 12.5 solution, prepared with NaOH pellets
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(97+ wt % pure) in deionized water, for 30 s at room temperature. This was again followed by 3 min rinsing with running deionized water and drying with compressed nitrogen. A set of samples was prepared by immersion in boiling deionized water for 60 s, followed by drying in an upright position. This pretreatment forms a so-called pseudoboehmite oxide layer, i.e., a thick aluminum hydroxide layer with excess water (AlOOH 3 H2O).23,24 The majority of this study concerns the characterization of the electropolished surface, modified with one of the four pretreatments (reference, acid, alkaline, or immersion in boiling water). In order to assess how the substrate condition affects the final properties of the modified surface, the above-described pretreatments were also applied on the as-received and electropolished + anodized surfaces. The term “substrate condition” (as-received, electropolished, or anodized) is used to describe the surface before the “pretreatment” (reference, acid-pretreated, alkaline-pretreated or immersion in boiling water). The as-received substrate condition represents the samples that are only ultrasonically cleaned in organic solvent prior to the pretreatments. The anodized substrate condition was formed by anodizing the electropolished (as explained in the previous paragraph) samples. Anodization was performed galvanostatically (5 mA/cm2) up to 150 V, in a solution of 0.1 M H3BO3 + 0.05 M Na2[B4O5(OH)4] 3 8H2O, with two platinum plates acting as cathodes. After rinsing and ultrasonic cleaning in deionized water (10 s and 2 min, respectively), they were blown dry using compressed nitrogen. In SKP measurements, humidity exposure was performed in situ at air ambient of 40% RH or 90% RH. XPS, VISSE, and FTIR measurements were performed ex situ, in which prepared samples were first exposed to humidity in an external humidity chamber and then immediately transferred into the equipment to be analyzed. At least three samples were analyzed for each parameter. SKP Study. Potential measurements were carried out with a commercial height-regulated SKP of KM Soft Control.25 The potential of the reference electrode was calibrated against a Cu/CuSO4 electrode in humid air at ∼90% RH.14 All the potentials were given relative to a standard hydrogen electrode (SHE). During the measurements, RH in the SKP chamber was kept constant at the desired level with an automated control system. For each sample, 100 data points were collected within a scan area of 2000 2000 μm with a step size of 200 μm. Tip material was NiCr (80/20) with the diameter of ∼125 μm. XPS Study. XPS spectra were acquired with a PHI 1600 instrument using a Mg K α radiation (1253.6 eV) X-ray source operated at 15 keV. The pressure in the analysis chamber was kept below 5 10 9 torr , and the analyzer pass energy was set at a constant value of 29.35 eV. The diameter of the analyzed area was 0.8 mm, and the samples were analyzed at 45 takeoff angle normal to the surface. Peak fitting was performed with CasaXPS software using a Shirley background and a mixed Gaussian Lorentzian shape (30%). Energy separation between each subpeak was kept constant. and the full width at half-maximum (fwhm) of each subpeak was kept in a certain range for all samples. The fitting parameters are given in the Supporting Information.3,21,26 VISSE Study. All VISSE measurements were obtained by a variable angle spectroscopic ellipsometer (VASE), J.A. Woollam Co. The wavelength ranged from 250 to 1000 nm in 10 nm steps. The angles of incidence used were 60, 65, and 70. The spectra were analyzed using the Complete EASE software (version 4.26) developed by J. A. Woollam Co. 1806
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Figure 1. Average potential values (vs SHE) obtained on differently pretreated surfaces of AA1050 (reference, acid, alkaline, pseudoboehmite) at 40% RH and 90% RH at (a) 40 min and (b) 240 min; inset is the time dependency of potential (vs SHE) obtained on differently pretreated surface at 40% RH.
FTIR Study. FTIR spectra were obtained by a Thermo-Nicolet Nexus FTIR apparatus which was equipped with a liquid-nitrogencooled MCT-A (mercury cadmium tellerium) detector and a SAGA grazing angle accessory at an incident angle of 80. Prior to collection of the spectra, an infrared background was obtained on a freshly electropolished aluminum substrate and the final spectra were ratioed against this background. For each spectrum, 64 scans with a resolution of 2 cm 1 were coadded.
’ RESULTS Figure 1 shows the averaged potential values obtained on the differently pretreated (oxyhydr)oxide surfaces upon exposure to 40% RH and 90% RH. From the measurements performed up to 400 min, it is observed that the potentials reach stable values after ∼200 min of exposure (Figure 1b inset). Because of such time dependency of the potentials, the data were analyzed for two snapshots for each humidity level: 1, average of the 5 measurements obtained within the first 90 min (referred as 40 min) (Figure 1a); 2, average of the 5 measurements obtained between 200 and 280 min (referred as 240 min) (Figure 1b). Considering the low range of error bars, obtained results are repeatable. It is clearly observed that different pretreatments result in different potentials. Among all the modified surfaces, pseudoboehmite surfaces exhibit the highest potential values of 0.15 0.05 V upon exposure to 40% RH and also 90% RH, respectively. For the samples which are exposed to 90% RH, a similar trend in the potential is observed for both 40 and 240 min snapshots, in which the potential of each pretreated sample increases in the order of
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Figure 2. Correlation of potential (vs SHE) obtained on differently pretreated surfaces (reference, acid, alkaline, pseudoboehmite) to the hydroxyl fraction obtained by XPS, at 240 min upon exposure to (a) 40% RH and (b) 90% RH.
acid-pretreated < reference ∼ alkaline-pretreated < pseudoboehmite. Samples exposed to 40% RH exhibit a slightly different trend in which the potential increases in the order of reference < acid-pretreated ∼ alkaline-pretreated < pseudoboehmite. It is also observed for both humidity levels that longer exposure results in more negative potential values for all surfaces except the pseudoboehmite ones in which the potential increases with time. Additionally, at 90% RH exposure, all surfaces, except the reference ones, result in more negative potentials than at 40% RH. In order to understand the observed trends in the potential, hydroxyl fraction, and oxide thickness, respectively, obtained by XPS and VISSE, were correlated to the potential values obtained by SKP (Figure 2 and Figure 3). Since the potentials reached stability 200 min after the start of the measurements, values recorded at 240 min are used for further correlation. Table 1 exhibits the hydroxyl fractions obtained by XPS on the modified AA1050 surface upon exposure to 40% RH and 90% RH. The surface hydroxyl fraction, obtained by the deconvolution of the O1s peak into single OH , O2 , and H2O components, is the peak area ratio of OH to O2 components.10,27 Since oxygen in O CdO and C O has the same binding energy as OH , fitting of the C1s signal was also performed and the contribution of C O and O CdO components was subtracted from the OH peak,3,10,21,26,27 in order to prevent the overestimation of the hydroxyl fraction caused by such overlap. Furthermore, the presence of water layer on the surface was also taken into consideration, and it was included in the peak fitting. Typical XPS peaks for O1s and C1s signals and their 1807
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Figure 4. Effect of various pretreatments (reference, acid, alkaline, and pseudoboehmite) on the potential (vs SHE) of as-received, electropolished, and anodized AA1050.
Figure 3. Correlation of potential (vs SHE) obtained on differently pretreated surfaces (reference, acid, alkaline, pseudoboehmite) to the oxide thickness obtained by VISSE, at 240 min upon exposure to (a) 40% RH and (b) 90% RH.
Table 1. Hydroxyl Fractions Obtained by XPS on Modified AA1050 Surfaces upon Exposure to Different Humidity Levels of 40% RH and 90% RH for 240 Min type of oxide layer
OH fraction 40% RH 240 min
OH fraction 90% RH 240 min
reference
0.57
0.49
acid-pretreated
0.52
0.47
alkaline-pretreated
0.58
0.52
pseudoboehmite
0.70
0.62
